Fig. 7.11 in VS .NET

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Fig. 7.11
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7.4 FUNCTION PROCEDURES A function procedure is similar to a sub procedure, with one important difference: a function is intended to return a single data item, just as a library function returns a single data item. Each function name therefore represents a data item, and has a data type associated with it. Within a function definition, the function name must be assigned the value to be returned, as though the function name were an ordinary variable. In its simplest form, a function procedure is written as
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Function procedure name (arguments) As data type . . . . .
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procedure name = . . . . .
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. . . . . End Function
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As with a sub procedure, the list of arguments is optional. Arguments represent information that is transferred into the procedure from the calling statement. Each argument is written as a variable declaration; i.e., argument name As data type Remember that the data type can be omitted if the argument is a variant. Multiple arguments must be separated by commas. If arguments are not present, an empty pair of parentheses must appear in the Function statement. The data type designation in the Function statement refers to the data item being returned. This designation is not essential the returned data item will be considered to be a variant if the designation is not included. Notice that the procedure name is assigned a value at some point within the procedure (multiple assignments are permitted, in accordance with the required program logic). This is the value being returned by the function. Thus, within a function, the procedure name is used as though it were an ordinary variable. (Contrast this with a sub procedure, where the procedure name does not represent a data item.)
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PROCEDURES
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[CHAP. 7
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EXAMPLE 7.5 DEFINING A FUNCTION PROCEDURE
Here is a function procedure that determines the factorial of a positive integer quantity. The function is based upon logic similar to that given in Example 4.11.
Function Factorial(n As Integer) As Long Dim i As Integer If n < 1 Then Beep MsgBox ("ERROR - Please try again") Else Factorial = 1 For i = 1 To n Factorial = Factorial * i Next i End If End Function
This procedure has one integer argument, n, which represents the value whose factorial will be determined. Thus, the value of n is transferred into the procedure, and its factorial is returned as a long integer. Note that the factorial is referred to by the function name, Factorial. Notice also that the function name (Factorial) is assigned a value at two different places within the procedure, as required by the program logic.
A function procedure is accessed in the same manner as a library function, by writing the function name and its required arguments as an expression. Thus, the function name (and its arguments) can be assigned to another variable, etc. The list of arguments in the function access must agree with the argument list in the function definition in number, in order and in data type. As with sub procedures, however, the names of the arguments in the function access may be different than the argument names used in the function definition.
EXAMPLE 7.6 ACCESSING A FUNCTION PROCEDURE
Let us now consider a complete Visual Basic program that determines the factorial of a positive integer n. The program will use the function procedure presented in the last example.
Fig. 7.12
CHAP. 7]
PROCEDURES
The layout of the form design window, shown in Fig. 7.12, is identical to that given in Example 4.11. However, the code is different, as shown below.
Function Factorial(n As Integer) As Long Dim i As Integer If n < 1 Then Beep MsgBox ("ERROR - Please try again") Else Factorial = 1 For i = 1 To n Factorial = Factorial * i Next i End If End Function Private Sub Command1_Click() Dim n As Integer, nFact As Long n = Val(Text1.Text) nFact = Factorial(n) Text2.Text = Str(nFact) End Sub Private Sub Command2_Click() Text1.Text = "" Text2.Text = "" End Sub Private Sub Command3_Click() End End Sub
Note the manner in which the function procedure Factorial is accessed within event procedure Command1_Click; i.e.,
nFact = Factorial(n)
Thus, the value of n is transferred into Factorial as an argument. The factorial of n is then returned by the function and assigned to the long integer variable nFact. The value of nFact is then converted to a string and displayed within text box Text2. In the above code, two separate statements are used to access Factorial and to display its returned value; i.e.,
nFact = Factorial(n) Text2.Text = Str(nFact)
This was done in order to clarify the program logic. The two statements can be combined, however, by simply writing
Text2.Text = Str(Factorial(n))
When executed, this program behaves in the same manner as the program shown in Example 4.11. The result obtained with a representative value of n = 11 is shown in Fig. 7.13.
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